The human NKG2A chain of the CD94/NKG2A receptor contains two immunoreceptor Tyr-based inhibitory motifs (ITIMs)in its cytoplasmic tail. To determine the relative importance of membrane-distal (residues 6-11) and membrane-proximal (residues 38-43) ITIMs in mediating the inhibitory signal, we made site-directed mutants of NKG2A at the Y (Y8F, Y40F, Y8F/Y40F) and the residues two positions N-terminal (Y-2) of Y (V6A, 138A, V6A/138A)in each motif. Wild-type (wt) and mutated NKG2A were then cotransfected with CD94 into rat basophilic leukemia 2H3 cells. Immunochemical analyses after pervandate treatment showed that each of the mutant molecules could be phosphorylated to expected levels relative to wt NKG2A and that all the mutations significantly reduced the avidity of SH2 domain-bearing tyrosine phosphatase-1 for NKG2A. Confocal microscopy was used to determine whether SH2 domain-bearing tyrosine phosphatase-1 and CD94/NKG2A colocalized intracellularly after receptor ligation. Only the Y8F/Y40F and Y8F mutant NKG2A molecules failed to show a dramatic colocalization. In agreement with this result, the Y8F/Y40F mutant was unable to inhibit FceRI-mediated serotonin release and the Y8F mutant was relatively ineffective compared with wt NKG2A. In contrast, the Y40F mutant was 70% as effective as wt in mediating inhibition, and the Y-2 mutations did not remarkable affect inhibitory function. These results show that, like KIR, both NKG2A ITIMs are required for mediating the maximal inhibitory signal, but opposite the KIR, the membrane-distal ITIM is of primary importance rather than the membrane-proximal ITIM. This probably reflects the opposite orientation of the ITIMs in type II vs. type I proteins.